1. Field of the Invention
This invention relates to the removal of multivalent metals, that is metals having more than one valence state, from waste effluent. More particularly, it relates to a process for the removal of iron and chromium in a form readily separated from the aqueous effluent using inexpensive raw materials.
2. Prior Art
Chromium is a noxious material and must be removed from waste effluent streams before such effluent can be discharged into rivers, streams, and the like. Various federal and state regulations now prevent certain metals from being discharged into waterways. Hexavalent chromium as a chromium ion, is the most common commercial form of chromium found in aqueous effluents. Heretofore, hexavalent chromium (Cr.sup.+6) has been converted to trivalent chromium (Cr.sup.+3). One such method is to use a reducing agent such as sodium bisulfite (NaHSO.sub.3) to form chromium sulfate Cr.sub.2 (SO.sub.4).sub.3, which is thereafter converted to chromium hydroxide Cr(OH).sub.3, and calcium sulfate (CaSO.sub.4) by the addition of calcium oxide. Cr(OH).sub.3 is difficult to remove from aqueous streams since it forms a gelatinous precipitate and calcium sulfate is also a fine precipitate and is hard to filter. U.S. Pat. No. 3,391,789 also discloses using SO.sub.2 to reduce Cr.sup.+6 to Cr.sup.+3 and the subsequent formation of Cr(OH).sub.3 by alkaline neutralization. In an attempt to overcome the defects with the beforementioned processes U.S. Pat. No. 3,493,328 uses a lead salt to react with the hexavalent chromium by forming lead chromate. Lead chromate however has a limited utility. Other U.S. Patents which disclose processes for recovering hexavalent chromium and other metal values are U.S. Pat. Nos. 3,575,854; 3,784,669; 3,810,542; and 3,819,051. In each instance, hexavalent chromium is reduced to the trivalent chromium which is the most stable form of chromium.
Various photographic and lithographic industries use aqueous solutions of dichromate-sensitized organic polymers such as polyvinyl alcohol that become insoluble upon irradiation with actinic rays. Additionally in the manufacture of printed circuit boards and color cathode ray tubes used for television such photosensitive organic solutions are used. In each instance, the polymers are exposed to a predetermined pattern of irradiation and the unexposed material is removed by water washes. The dilution of the dichromate-sensitized organic polymer aqueous solution makes it unsuitable for reuse or recycling. The diluted material, however, cannot be discharged without the removal of the chromate. The organic contained in the effluent when the chromium is converted to chromium hydroxide (Cr(OH).sub.3) makes the precipitate more gelatinous and more difficult to remove.
Another multivalent metal, iron, when it is in its highest valence state forms an extremely finely divided and gelatinous water-insoluble hydroxide. Because of the physical form of ferric hydroxide and chromium hydroxide, they are difficult to remove from the aqueous effluent using conventional filtration techniques. A method to remove these metals more readily would be an advancement in the art.